This invention relates to composite materials having artificially dispersed nano-size phases. More particularly, this invention relates to methods for manufacturing such materials using solidification processing techniques. This invention also relates to articles made using such methods.
The term “composite material” as used herein generally refers to a class of materials comprising a combination two or more different materials, for example, tungsten carbide particles dispersed within a cobalt alloy. Composite materials often comprise a discontinuous or fibrous phase dispersed within a matrix phase. The functions served by the various phases are manifold and depend on the application for which the composite material is intended. For example, many composites designed for enhanced mechanical properties comprise a hard, strong discontinuous phase, such as, but not limited to, particles, dispersed within a more ductile matrix phase, such as a metal. The matrix phase serves to bind the particles to the material and to provide toughness and impact resistance, while the particles add hardness, wear resistance, and strength to the material. Typically, as the space between particles (referred to as the interparticle spacing) becomes closer, the resultant material becomes stronger. A dispersion of close particles restricts dislocation movement, and thus strengthens the material.
FIG. 1 is an exemplary graph of particle strengthening effects, showing an increase in the Orowan shear stress with a decrease in the interparticle spacing (IPS), in nanometers. This figure represents a generalized relationship of strength versus IPS, using Orowan stress T (MPa) that is calculated from the equation: T={Gb ln(D/b)}/{2π(T−D)}, where G is shear modulus, b is Burger's vector, D is the diameter of the particle, and T is the interparticle spacing.
Consistent with the mechanical strength example described above, materials with microstructural features such as grains or discontinuous phases with sizes on the order of about 100 nm and less have shown a wide variety of desirable properties. There is considerable interest in the class of materials known as “nanocomposites,” composite materials that comprise a dispersion of at least one nano-scale discontinuous phase within a matrix phase. As used herein, the terms “nano-scale” and “nano-size” both refer to materials having an average size of about 100 nm or less in at least one dimension. The high percentage of atoms residing at interfaces within nanocomposite materials, along with the multitude of potential matrix/particle material combinations, creates the potential for unprecedented material properties and combinations of properties.
Several technological challenges are evident in the manufacture of nanocomposite materials. While solidification processing of metal-matrix composites is common in the art, the average size of the dispersed phase is generally well over 100 nm, and thus such composites cannot take advantage of the unique benefits offered by nano-scale materials, as exemplified by the effects documented in FIG. 1. There is a longfelt need in the materials industry for nanocomposites manufactured via solidification processing, due to the advantageous cost and flexibility offered by this type of processing. However, the behavior of materials becomes dramatically different as their size is reduced from the micron and sub-micron scales (that is, greater than 100 nm) to the nano-scale (that is, 100 nm or less), due in part to the much higher surface areas per unit of weight of nano-sized materials versus larger reinforcements. Production of nanocomposites has been shown not to be simply an adaptation of processes used to form composites having micron-sized and larger phase dispersions. Typical methods used in the art for dispersing micron-scale phases in molten matrix materials can result in the non-uniform distribution of nano-scale phases, due to the greater tendency of the nano-scale phases to agglomerate, float, sink and combinations thereof. Experimental work performed by A. M. Tissier and J. K. Tien (Metallurgical Transactions A, 21 A (March, 1990), pp. 753-755) demonstrates the agglomeration problem in detail and the difficulties posed by this phenomenon. Furthermore, other researchers (P. Busse et al., Journal of Crystal Growth, 193 (1998), pp. 413-425) have not only demonstrated the difficulties due to nano-size phase agglomeration, but have also speculated that, in general, it is not possible to maintain a stable homogeneous suspension of ceramic nano-scale phases in molten metal, thus rendering efforts to make single-crystal nanocomposite materials via solidification processing futile.
Composites produced by mechanical alloying and their associated formation processes are also known in the art. Mechanically alloyed particle-dispersion strengthened articles are made using powder metallurgy (PM) processes. The PM processes include, but are not limited to, hot isostatic-pressing (HIP) processes. PM processes have inherent size limitations in which PM production is limited to relatively small articles (those articles that have a diameter less than about 20 centimeters). PM processes are impractical for dispersion strengthening of large metal articles, such as large power generation equipment including rotors for steam turbines. In addition, PM processes result in materials that are significantly higher in cost compared to materials processed using solidification processes such as casting.
Therefore, there is a need to provide methods to efficiently and effectively manufacture nanocomposite materials in a cost-effective manner. Furthermore, there is a need to provide articles made from such technologically attractive and cost-effective materials.